I grew up baptist, and the first time I ever doubted anything was in 7th grade, when an atheist classmate said there’s no way that Noah’s Ark could be true.
But, after that, in my later high school years, I got REALLY into church. I started going to a Pentecostal church and believed that everything in the Bible was literally true.
It wasn’t until when I was 26 (about 10 years ago), when a coworker asked me why I believe, that I started really questioning. He was an ex-preacher who went to college and learned about other religions. He made the point that where you are born largely dictates what you believe.
A light bulb went off for me, and it all made so much sense. Religious indoctrination is why most people believe. It didn’t take long for me to completely deconvert after that.